This invention relates in general to a thin, surface-type illumination device for providing backlight for liquid crystal displays (LCD) and, in particular, to a suitable illumination device for use in a notebook computer display that provides high brightness with low power consumption as well as to a liquid crystal display that uses this illumination device.
Surface-type illumination devices with a cylindrical light source and a flat light guide plate such as the devices described in Japanese Laid-Open Patent Application No. 60-205576 and Japanese Laid-Open Patent Application No. 61-248079 are well-known. One such example is shown in FIG. 21. In the illumination device 90, cylindrical fluorescent light 92 is positioned on one side of the substantially rectangular and flat light guide plate 91. The light introduced to light guide plate 91 from fluorescent light 92 is reflected by the diffusion pattern printed on light guide plate 91 and emitted from the surface of the light guide plate at a fixed density of light.
This type of surface illumination device, in recent years, has been used extensively to provide backlight for liquid crystal display panels. Liquid crystal display panels are increasingly used as displays in such devices as laptop computers, televisions and cameras. The use of liquid crystal display panels for color displays is also increasing. As the size of personal computers and televisions become smaller, it is imperative that liquid crystal display panels become thinner and lighter.
Accordingly, it is necessary that the surface-type illumination device used for liquid crystal display panels to provide backlight, along with the color displays, becomes thinner and lighter with less power consumption. Also, to use in color displays, a sufficient brightness is necessary to clearly show the colors displayed in the liquid crystal. This requires the use of a high output fluorescent light in the illumination device. However, along with the light, heat is also radiated from the high-output fluorescent light. The effect of this heat, as shown in FIG. 22, is significant. The temperature may rise 30-40.degree. C. above a normal temperature of 25.degree. C. Consequently, when this type of illumination device is used for providing backlight in an MIM active color display panel or in an STN passive color display panel, a special method to reduce the heat is necessary to control, to a certain extent, the color and brightness irregularities.
Instead of using one high output fluorescent tube, one may also increase the number of fluorescent tubes. In this way, it is possible to control to some extent the temperature increase due to the light source. However, as the number of fluorescent tubes is increased, many other problems appear. One of these problems is the variations in the illumination of the fluorescent tubes. Because the illumination intensity of florescent tubes varies according to each tube, it is necessary to adjust such things as the resistance within the fluorescent tube driver circuit to obtain a fixed illumination intensity. Consequently, in the case when several fluorescent tubes are used in one illumination device, extra time is required during the manufacturing process to obtain a balanced and fixed illumination intensity.
Another problem is the increased number of driver circuits required to turn on the fluorescent tubes. The number of these driver circuits can not be easily increased in devices such as microcomputers where thinness and small size are important.